1. Field of the Invention
The present invention relates to a cable equalizer used in receivers for digital baseband transmission communication and the like, and more particularly to an adaptive cable equalizer suitable for the High-Definition Multimedia Interface (HDMI).
2. Description of Related Art
In recent years, HDMI for connecting digital televisions, digital versatile disk (DVD) devices and the like with high-speed digital baseband communications has become widely used. HDMI refers to communication using Transition Minimized Differential Signaling (TMDS), which is a method of digital baseband transmission in differential current mode. Communication mainly is performed using three TMDS data lines and one TMDS clock line, enabling communication at data transfer rates of approximately 200 Mbps to 3 Gbps.
The transmitting side (source device) of HDMI converts parallel data to serial data using a serializer, based on parallel data and a clock synchronized with the parallel data, and simultaneously sends the serial data to a TMDS data line and the clock to a TMDS clock line. The transfer rate of the clock is standardized to 1/10th of the serial data. The receiving side (sink device) of HDMI converts TMDS data to parallel data using a deserializer that performs clock data recovery (CDR) based on the TMDS clock, and demodulates the data.
Heretofore, the HDMI standards (v1.2) supported communication of 24-bit pixel data at data transfer rates up to 1.5 Gbps, although a standard (v1.3) was formulated in 2006 enabling communication of maximum 48-bit pixel data at 3 Gbps.
Cable equalizers that enable the amount of equalization to be varied are provided on the receiving side following the increases in data transfer rates described above, and technology has been proposed to increase the communicable cable length to approximately 30 meters by employing adaptive cable equalizers that automatically adjust the equalization amount of the equalizer according to the reception state.
Hereinafter, adaptive cable equalizer technologies disclosed in the specifications of U.S. Pat. Nos. 6,819,166 and 6,584,151 and in “An Adaptive Cable Equalizer for Serial Digital Video Rates to 400 Mb/s” (ISSCC 1996, Session 10, Paper FA 10.7) will be described. It is well known that with digital database transmission, the attenuation characteristics of a cable deteriorate relative to a frequency as the cable length increases. Not only does the amplitude of data transmitted from the transmitting side decrease, but there is an increase in the transition time of data from high to low or from low to high. A drop in the slew rate at which data changes leads to an increase in data jitter when separating data with CDR, and a rapid deterioration in the bit error rate (BER).
Generally, the transition time characteristics of data signals are evaluated using an eye pattern, with the eye opening collapsing as cable length increases. In view of this, with the technologies disclosed in the above documents, an adaptive cable equalizer such as shown in FIG. 9 is installed on the receiving side, the equalization amount of the variable equalizer is controlled by measuring the transition time of received data from high to low or from low to high, and the eye is controlled to open so that communication quality is ensured.
With the adaptive cable equalizer in FIG. 9, reference numeral 1 denotes a data signal input unit, and reference numeral 2 denotes a clock signal input unit. A data signal input from the data signal input unit 1 is input to a variable equalizer 3, and a data signal output from the variable equalizer 3 is input to a transition time measuring portion 4, where the transition time of the data signal is measured. The output signal of the transition time measuring portion 4 is processed by an integrator 15 to obtain a control signal, and a control loop is configured by controlling the variable equalizer 3 using the control signal.
The variable equalizer 3 is constituted by a differentiator 6, a variable gain amplifier 7 and an adder 8. The characteristics of the variable equalizer 3 change as a result of the gain of the variable gain amplifier 7 changing according to the output signal (control signal) of the integrator 15.
The transition time measuring portion 4 is constituted by a quantizer (comparator) 9, differentiators 10a and 10b that are constituted by high-pass filters and the like and to which the input and output signals of the quantizer 9 are input, full-wave rectifiers 11a and 11b that perform full-wave rectification on the differential signals of the differentiators 10a and 10b, and a subtractor 12 to which the output signals of the full-wave rectifiers 11a and 11b are input. The output signal of the subtractor 12 is input to the integrator 15, which is constituted by a low-pass filter that includes a capacitor 16 and a resistor 17. The variable equalizer 3 is controlled using the control signal output from the integrator 15, so that the transition times of the input and output signals of the quantizer 9 are the same.
FIG. 10 shows the signal waveforms of the elements in the adaptive cable equalizer of FIG. 9. (a) and (b) are the input and output signals of the quantizer 9, which are respectively input to the differentiators 10a and 10b. With the output of the differentiators 10a and 10b, the gradients of the transition times from high to low or from low to high are output as the differential signals of the differential waveforms (c) and (d). Full-wave rectification is performed on the differential waveforms (c) and (d) by the full-wave rectifiers 11a and 11b to obtain the signals (e) and (f). The subtractor 12 inputs the signal (g) obtained by subtracting the full-wave rectified signals (e) and (f) to the integrator 15. The integrator output (h) output from the integrator 15 is input to the variable equalizer 3 as a control signal, and control is performed so that the transition times of the input and output signals of the quantizer 9 are the same. (i) indicates the clock signal.
The technology disclosed in the above documents enables communication quality to be ensured using this method, even when cable length increases.
However, with conventional adaptive cable equalizers, the response time of the control loop is decided primarily by the time constant of the integrator 15, and moreover the response time of the control loop is fixed. Therefore, there is not a constant relationship between the time needed for the control loop to stabilize and the data bit number of transferred data.
Because HDMI data bit strings are modulated for CDR so as to contain no consecutive 0's or 1's, the distribution of data frequency components has an approximately 20-fold frequency distribution, from 1010 to 100 . . . 01. Further, the data transfer rate needs to be variable from approximately 200 Mbps to 3 Gbps to match the display format.
Consequently, in the case of the time constant of the integrator 15 being fixed, settings such as shown in FIGS. 11A and 11B are required. The waveforms (g), (h) and (i) shown in FIGS. 11A and 11B correspond to the waveforms (g), (h) and (i) in FIG. 10. FIG. 11A shows the case of a slow data transfer rate, while FIG. 11B shows the case of a fast data transfer rate. The time constant of the integrator 15 needs to be set sufficiently longer than approximately 1/20th of the minimum transfer rate, so as to enable peak detection of the data transfer rate of the subtractor 12, even at the minimum transfer rate. If the time constant of the integrator 15 is not set sufficiently long, the integrator response will be insufficient, as shown by the integrator output (h) of FIG. 11A.
On the other hand, when the time constant of the integrator 15 is set to meet this condition, the data bit number needed for the control loop response of the adaptive cable equalizer to stabilize at the maximum transfer rate is extremely large. Further, integration into a semiconductor apparatus is also difficult, since the time constant of the integrator 15 often is set using a capacitor, and the capacitance value for obtaining a response time that meets the above condition is extremely large.